This invention relates to a racket for use in tennis, squash and similar games, provided with a novel stringing pattern which not only improves the performance of the racket in play, but also increases the durability of the frame by distributing the forces created by the tension of the strings in a more uniform manner, thus tending to decrease the likelihood of breakage.
Although most tennis and similar rackets continue to be strung in the traditional manner, i.e., with a plurality of parallel main strings extending generally parallel to the longitudinal axis of the handle and a plurality of parallel cross-strings arranged perpendicularly to the main strings, there have been a number of proposals in which this pattern has been modified, with the objective of improving the performance of the racket, or increasing the durability thereof, or both. It is known, as exemplified in U.S. Pat. Nos. 644,877, 3,820,785, 3,999,756 and 4,013,289, to increase the effective resiliency of the stringing by increasing the relative density of the strings in the central portion of the striking surface, either by adding additional main or cross strings to a uniformly strung pattern, or by eliminating some of the main and/or cross strings farthest from the center of the striking surface, i.e., those closest to the frame. Although such non-uniform stringing patterns have been found to improve the playing characteristics of the racket in certain desirable ways, as for example by increasing the size of the "sweet spot", i.e., the area in which contact of the strings with the ball produces a minimum jarring impact on the arm of the user, they also present certain practical disadvantages. In those rackets in which increased density in the center of the striking surface is achieved by removing peripheral strings, the remaining strings must have increased tension and are subject to increased wear, leading to early failure. On the other hand, when additional strings are used to increase the density in the center of the striking surface, particularly in the case of rackets having wooden frames provided with holes through which the strings pass, the closely spaced holes required to accommodate the additional strings weaken the frame and lead to premature failure in play.
As a solution to one of the problems noted above, i.e., the weakening effect on the frame of a wooden racket created by the holes in the frame required for stringing, there have been proposals, exemplified by U.S. Pat. Nos. 455,632, and 4,118,029, British Pat. Nos. 6475 and 300,700 and French Pat. Nos. 550,985 and 855,902, to pass the ends of an adjacent pair of main or cross strings through single holes in the frame, thereby reducing the total number of holes required to accommodate a given number of main or cross strings. In French Pat. Nos. 855,902, it has been proposed to eliminate the use of holes entirely and to wrap the strings continuously around the outer circumference of the frame in a zig-zag fashion, the strings being maintained in proper location by grooves on the outer circumference of the frame, adjacent strings being connected to a single attaching point at one of their respective ends and to different attaching points at their other ends. These proposals similarly present practical difficulties which have militated against their widespread use, among them being difficulty in stringing the racket and the necessity of using overlapping double lengths of stringing material between some adjacent holes on the outer surface of the frame. Not only does such stringing create areas of unequal stress in the frame, but the groove commonly provided in the outer surface of the wooden racket to receive the connecting lengths of stringing material, intended to avoid contact of the strings with the ground, also must be made twice as large, thereby further weakening the frame at these points. In addition, the overlapping of the interconnecting lengths of stringing material tends to crush the material, thus tending to reduce its durability and to lead to premature failure.
U.S. Pat. Nos. 3,086,777 and 3,206,203 disclose rackets employing a frame made of steel tubing which is unperforated and therefore not weakened by the presence of holes for securing the strings. In the rackets disclosed in these patents, the strings are secured to a serrated annular member or crown which is itself attached to the inner periphery of the frame by a resilient member such as a helical wire. While such tennis rackets have met with substantial commercial success and have been marketed in large numbers, there has been some dissatisfaction with the resiliency of the strung surface. Attempts to improve the resiliency of such rackets by providing additional string density in the center of the strung surface, in accordance with prior suggestions, have presented practical problems, in that the stiff wire forming the crown is not sufficiently deformable to permit the formation of additional serrations to allow additional strings.